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3-5
3.6 SART
Note: The SART (Search and Rescue
Transponder) information below is
excerpted from IMO SN/Circ 197
Operation of Marine Radar for SART
Detection.
A Search and Rescue Transponder
(SART) may be triggered by any X-Band (3
cm) radar within a range of approximately
8 nm. Each radar pulse received causes it
to transmit a response which is swept
repetitively across the complete radar
frequency band. When interrogated, it first
sweeps rapidly (0.4 µs) through the band
before beginning a relatively slow sweep
(7.5 µs) through the band back to the
starting frequency. This process is
repeated for a total of twelve complete
cycles. At some point in each sweep, the
SART frequency will match that of the
interrogating radar and be within the pass
band of the radar receiver. If the SART is
within range, the frequency match during
each of the 12 slow sweeps will produce a
response on the radar display, thus a line
of 12 dots equally spaced by about 0.64
nautical miles will be shown.
When the range to the SART is reduced to
about 1 nm, the radar display may show
also the 12 responses generated during
the fast sweeps. These additional dot
responses, which also are equally spaced
by 0.64 nm, will be interspersed with the
original line of 12 dots. They will appear
slightly weaker and smaller than the
original dots.
9500 MHz
9200 MHz
Radar antenna
beamwidth
Screen A: When SART
is distant
Screen B: When SART
is close
Lines of 12 dots
are displayed in
concentric arcs.
Echo of SART
Position of
SART
Own ship's
position
Own ship's
position
SART mark
length
Radar receiver
bandwidth
Sweep time
7.5 µs
95 µs
Sweep start
High speed sweep signal
Low speed sweep signal
24 NM
1.5 NM
Position of
SART
Echo of
SART
Figure 3-8 Appearance of SART signal
on the radar display
General procedure for detecting
SART response
1. Use the range scale of 6 or 12 nm as
the spacing between the SART
responses is about 0.6 nm (1125 m) to
distinguish the SART.
2. Turn off the automatic clutter
suppression.
3. Turn off the Interference Rejector.
General remarks on receiving
SART
SART range errors
When responses from only the 12 low
frequency sweeps are visible (when the
SART is at a range greater than about 1
nm), the position at which the first dot is
displayed may be as much as 0.64 nm
beyond the true position of the SART.
When the range closes so that the fast
sweep responses are seen also, the first of
these will be no more than 150 meters
beyond the true position.